Solder glass and electrical device employing the same

ABSTRACT

A solder glass comprised of about 65 weight percent Sb 2  O 3  ; about 30 weight percent B 2  O 3  and about 5 weight percent V 2  O 5 . The glass is black and extremely visible when employed as a molten seal in electrical devices such as tungsten-halogen lamps.

TECHNICAL FIELD

This invention relates to solder glasses and more particularly to suchglasses which become molten at about 350° C. and can be employed withquartz-to-metal seals in electrical devices.

BACKGROUND ART

Certain electrical devices, e.g., tungsten halogen lamps, employenvelopes comprising a high percentage of silica, such as fused silica,fused quartz or Vycor, the latter being a 96% silica glass. To achieve ahermetic seal between the glass and the lead-in conductors of theelectrical device it is common practice to employ a pinch seal. Thelead-in conductors generally comprise a very thin foil portion ofmolybdenum which forms the actual hermetic seal and an outer lead-wireportion of a refractory metal which can also be molybdenum. One end ofthe lead-wire is attached to the foil and the other end extendsexteriorly of the pinch. This relatively heavy outer lead-wire does notform a hermetic seal with the quartz because of its relatively largediameter and the difference in thermal expansion coefficients betweenthe two. In actual practice, because the formation of the pinch sealdoes not allow the quartz to flow completely around and against the fullperiphery of the outer lead-wire, a small capillary passage is lefttherebetween. The thin foil is thus exposed to atmospheric oxygenentering via the capillary passages. At elevated temperatures, say aboveabout 350° C., oxidation of the foil can occur, resulting in breakage ofthe electrical connection to the outer lead-wire.

It has been suggested that this problem can be reduced by filling thecapillary with a solder glass which becomes molten when the device isoperated, thus forming a molten seal. Specifically, a lead borate glasshas been suggested; however, the use of such a glass requires the use ofplatinum or platinum clad lead-wires since lead borate attacksmolybdenum. Further, U.S. Pat. No. 3,588,315 suggests binary glassessuch as antimony borate and ternary glass compositions of antimonyborate with the addition of small amounts of molybdenum trioxide ortungsten trioxide.

It would be an advance in the art if additional solder glasses could bemade available. It would be a further advance if such glasses wereopaque to visible radiation.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to provide a solder glasshaving a melting temperature at about 350° C. and which is notdeleterious to molybdenum.

These objects are accomplished, in one aspect of the invention, by theprovision of a solder glass comprised of Sb₂ O₃, B₂ O₃ and V₂ O₅. Thisglass has the requisite melting temperature, is not deleterious tomolybdenum and is opaque to visible radiation. Thus, as a seal forelectrical devices, it easily fills the capillary passages and itspresence is easily monitored because of its visibility (it is black).

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a partial, sectional view, somewhat exaggerated asa size, of the end of an electrical device employing the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawing.

Referring now to the drawing with greater particularity there is shown aseal for an electrical device 10, which can be a tungsten-halogen lamp.Only a single end of the device 10 is illustrated, it being understoodthat in a double-ended device the end not shown would be similar. It isalso to be understood that the invention is equally applicable tosingle-ended devices wherein a plurality of lead-in connectors areadjacent to one another. Device 10 has a body 11 of substantially fusedsilica or quartz or some other high silica content glass. By high silicacontent glass is meant a glass having more than 90% silica.

At least one electrically conductive member 12 is sealed in the end ofdevice 10. The member 12 comprises a proximal portion 14 which projectswithin the body 11, an intermediate thin foil portion 16, as of amolybdenum which forms the hermetic seal with body 11, and a distalportion 18 which projects exteriorly of the body 11.

As noted above, the formation of such a pinch seal leaves a smallcapillary passage 20 surrounding the distal portion 18. The passage 20may have a width on the order of 0.001 inch.

To prevent oxidation of the foil 16 at elevated temperatures, i.e.,above 350° C., the capillary passage 20 is filled with a solder glass22. The glass 22 comprises about 65 weight percent Sb₂ O₃ ; about 30weight percent B₂ O₃ ; and about 5 weight percent V₂ O₅. This solderglass becomes molten at about 350° C. and thus protects the foil 16.

This particular solder glass has the additional advantage that it isopaque to visible radiation, i.e., it is black; thus providing a visualindication of its presence. Also, because it is black, it is absorptiveof infrared radiation and, thus, may reach its molten condition soonerthan other solder glasses.

The glass is formulated by weighing out the antimony trioxide (Sb₂ O₃)and the vanadium pentoxide (V₂ O₅). The boron trioxide (B₂ O₃) issupplied from boric acid (H₃ BO₃) at the rate of 1.776 grams of the acidfor each gram of oxide desired. The materials are thoroughly mixedtogether and then melted. The melting is preferably carried out in asuitable crucible, such as one of kyanite (Al₂ O₃ --SiO₂), pure aluminaor platinum with 10% rhodium. The melts can be made by introducing thebatch, in small portions, into a crucible heated to about 850° C.Additional batch is added after the initial melting reaction beginsuntil the crucible is full. The melts were made in air. The melt is heldat the melting temperature of 850° C. for about fifteen minutes after itappears batch free and is then stirred with a fused quartz rod. Thetemperature is then lowered about 50° C. and held for about fifteenminutes and the melt is then cast into thin rods or washers.Alternatively, the viscosity of the melt could be controlled and rodsdrawn directly from the crucible.

The solder glass so made can be applied to the seal by positioning thedevice 10 vertically with the capillary to be filled uppermost. The body11, at the seal area, is heated and the solder glass is applied theretoand caused to run down into the capillaries. If the device isdouble-ended, it is then inverted and the procedure repeated.

While there have been shown what are at present considered to bepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madeherein without departing from the scope of the invention as defined bythe appended claims.

We claim:
 1. A seal for electrical devices comprising a body of substantially fused silica and at least one electrically conductive member sealed therein, said member comprising a proximal portion projecting within said body, an intermediate thin foil portion hermetically sealed within said body, and a distal portion projecting exteriorly of said body, and a capillary passage surrounding said distal portion and extending to said intermediate portion, said capillary passage being substantially filled with a solder glass that is opaque to visible radiation and is not deleterious to molybdenum, said solder glass forming, at elevated temperatures above about 350° C., a molten seal which prevents oxidation of said thin foil portion said solder glass comprising Sb₂ O₃, B₂ O₃ and V₂ O₅.
 2. The seal of claim 1 wherein said solder glass is black.
 3. The seal of claim 2 wherein said solder glass comprises about 65 weight percent Sb₂ O₃ ; about 30 weight percent B₂ O₃ ; and about 5 weight percent V₂ O₅.
 4. A solder glass comprising, by weight: about 65% Sb₂ O₃ ; about 30% B₂ O₃ ; and about 5% V₂ O₅. 